This invention relates in general to vacuum vapor deposition and in particular to a new and useful method and apparatus for vacuum vapor deposition with an electric arc discharge.
With the use of plasma-supported vacuum coating processes the spectrum of compounds, which can be generated reactively by supplying the requisite components from different sources in the deposition on a substrate, was considerable expanded beyond the wide-spread production of oxides. Most often a required metal component is introduced into the deposition chamber by vaporization or by sputtering, while the second or further components are admitted in the gaseous state.
Metal vaporization from hot spots (cathode points) is well suited for plasma-supported coating, since the hot spots emit the cathode material essentially not as neutral vapor, as other sources used for vacuum coating do, but for a major part as ions. These form together with the emitted electrons a plasma of high charge carrier density, which permits coating at a high rate. Simultaneously, through the plasma-supported layer formation favorable layer properties such as high density, good strength, little roughness are achievable and that for layers. The layers are well as, in particular for layers of compounds of this vapor with added "reactive gases, so that on the substrates a layer of the corresponding compound originates.
With planar cathodes the direction distribution of the emitted ions obeys nearly the cos law, that is, the stream of vaporized metal observed in a direction, which .alpha. with the perpendicular on the cathode surface includes angle .alpha.-- is smaller by the factor cos .alpha. than the emission in the perpendicular. This intensity distribution also applies approximately to the emissions of other known vapor sources for vacuum coating. Consequently, from the view point of rational coating it is expedient, to arrange the substrate to be coated near the perpendicular assigned to the vaporizing area of the source. When vaporizing from hot spots this is recommended for another reason also.
In contrast to other sources, the material of so-called hot spots is emitted from the surface of the cathode to a significant extent in the form of small liquid droplets in a direction distribution different from the ions, which in flight or upon impinging on the substrate solidify and in this way lend diverse disadvantageous properties to the layer. These properties include increased roughness of the layer surface, contamination of layers which are intended to be formed as chemical compound with metal vapor, increased roughness of the layer surface, contamination of layers which are intended to be formed as chemical compound with metal vapor, increased corrosion susceptibility after the spatters have been removed, and others. The least spatters are found on substrates, which are arranged in the vicinity of the perpendicular, that is where at the same time the coating rate is at a maximum.
Arrangements customary until today for vacuum coating using vaporization from hot spots, utilize these different emission characteristics of vapor and spatter in order to obtain layers with the least possible number of errors through spatters. For this purpose the substrates are arranged near each other in a small region relative to the diameter of the cathode or at a great distance from the cathode so that the mid-perpendiculars constructed on the cathode areas are directed toward the center of the individual substrates. However, the great spacing distance of the cathodes required of the substrates in this arrangement, and the small number of substrates which can be placed near the perpendiculars, does not permit cost-efficient low-spatter coating.
Therefore, when using the so-called spark vaporizer, for the sake of cost-efficient fabrication, compromises in the direction toward smaller distance respectively to more extensive substrate fields have been accepted in the bargain, which, however, carried with it substantial worsening with respect to spatter. Different known attempts are aimed at decreasing spatter without concurrent impairment of cost-effectiveness. It is known, that the number of spatters can be decreased, if the presence (dwelling time) of the hot spot at the particular site of the cathode is only brief. However, switching off the current and re-igniting after a short interruption as suggested for this purpose is not compatible with coating on a commercial scale.
It is known that the number of simultaneously active hot spots is a function of the current available for the cathode sputtering discharge, with a hot spot-- depending on the cathode material-- emitting a current between 50 and 150 A. The hot spots move statistically back and forth over the cathode surface. Through suitable magnetic fields they can be limited to moving within a selected region. Through a strong magnetic field directed parallel to the surface of the cathode, which, however, would need to change its direction in the plane, short dwelling times of the hot spots could also be achieved. Such procedure, would, however, be cumbersome and would not gain acceptance by the industry.